Airbed or alternative?

From now on, my boyfriend will stay with me one night a week. I'm looking for a cheap solution so we can have a double bed when he stays over. For the moment we share my single bed, but a little more space would be welcomed. A real bed is not really an option, it would be too expensive for one, and i prefer to sleep in a single when i'm alone. I live in a dorm, but there's plenty of space (32m�) so it does not need to be that compact when it's stored. I was thinking about an airbed, but doesn't that wobble like crazy?